Gas-Phase and Condensed-Phase Synergy in a Nonflammable Electrolyte for Highly Stable Sodium-Ion Batteries

Mitigating electrolyte-induced thermal runaway to ensure safety while preserving superior electrochemical performance remains a critical challenge for secondary batteries, especially in electric vehicle applications. Herein, we design a synergistic electrolyte with trimethyl phosphate (TMP) and ethoxy-pentafluorocyclotriphosphazene (PFPN) as additives in a conventional commercial carbonate electrolyte, enabling dual gas-phase and condensed-phase flame retardancy via functional zone partitioning in the electrolyte solvation structure. This nonflammable electrolyte simultaneously enhances the battery safety and electrochemical properties. The Na0.75Li0.15Mg0.05Ni0.1Mn0.7O2||HC full cell delivers an improved specific capacity of 102.9 mAh g–1 at 0.02 A g–1 and a capacity retention of 75% after 1000 cycles at 0.1 A g–1. The capacity retention rate of a single-stack pouch cell is 80% after 500 cycles, which also proves the applicability of the electrolyte. Specifically, TMP preferentially coordinates in the inner solvation sheath to suppress gas-phase flame propagation via radical scavenging, which is confirmed by differential electrochemical mass spectrometry (DEMS) through its effective suppression of H2 release and interruption of the combustion chain reaction. In contrast, PFPN achieves condensed-phase flame retardancy by forming a dense protective carbon layer at high temperatures, with in-depth X-ray photoelectron spectroscopy (XPS) analysis revealing that P–N/P–F functional groups enable it to act as both a “fire barrier” and an “interfacial stabilizer”, thereby preventing continuous decomposition of flammable components. This work provides novel insights for flame-retardant electrolyte design.